(1) Field of the Invention
The present invention relates to a SiC crystal and to a semiconductor device, and in particular relates to a SiC crystal comprising acceptor impurities and donor impurities.
(2) Description of the Related Art
In recent years, light-emitting diodes have come to be widely used as light-emitting elements fabricated using compound semiconductors, and are used primarily for light transmission, display, and for special lighting and other applications. Moreover, white-light light-emitting diodes have been commercialized which use blue-light light-emitting diodes employing nitride semiconductors to cause emission of blue light, which is passed through a YAG yellow phosphor to generate white light. In YAG yellow phosphors, a portion of the blue light is converted into yellow light. The converted yellow light and the unconverted blue light then undergo color mixing, so that white light can be obtained. Such a white-light light-emitting diode can also be used in ordinary lighting, and is expected to find uses in various applications.
However, there has been the problem that the energy conversion efficiency in the above-described white-light light-emitting diodes is inadequate compared with existing fluorescent lamps. Further, because white light is composed from only a blue component and a yellow component, the red component is inadequate, and there has been the problem that color rendering as an illumination light source is inferior. A method has been proposed for improving color rendering by combining a red phosphor with a yellow phosphor; but at present, the conversion efficiency of red phosphors is low, and it has been impossible to achieve both satisfactory color rendering and adequate energy conversion efficiency. Moreover, there has been the problem that, if the film thickness and concentration of the yellow phosphor are not uniform, yellow or blue colors may appear depending on the light-emitting site.
On the other hand, SiC phosphors, in which SiC crystals are doped with acceptor impurities and donor impurities, have been proposed (see for example International Patent Publication No. PCT/JP2005/090515). In this configuration, through stimulation by near-ultraviolet light emitted from a near-ultraviolet light-emitting diode or similar, donor electrons and acceptor holes can recombine, and the energy transition upon this recombination results in light emission. Because the wavelength of this fluorescence depends on the energy gap between the donor level and the acceptor level, by appropriately choosing the elements introduced as the acceptor impurities and donor impurities, fluorescence at an arbitrary color can be induced.
In International Patent Publication PCT/JP2005/090515, by providing a two-layer SiC phosphor in which SiC crystals are respectively doped with boron and aluminum as acceptor impurities, to induce yellow and blue fluorescence, by mixing of these fluorescent colors, white light is obtained. The wavelengths of fluorescent light caused by recombination of donor electrons and acceptor holes extend over a comparatively broad wavelength range, despite the peaks at the yellow and blue colors, and so white light with satisfactory color rendering can be obtained. Further, in the process of light-emitting diode layering from an SiC substrate, SiC phosphors can easily be formed, so that there is no need to subsequently add phosphors alone.
However, there have not been adequate studies of the extents of doping with acceptor impurities and donor impurities which are optimal in order to realize high energy conversion efficiency in SiC phosphors. In particular, the relative relation between acceptor impurities concentrations and donor impurity concentrations has not been clarified, and so there has been the problem that high energy conversion efficiencies could not be realized.